emacs: devel: Highlight 'modify-phases' keywords.

* emacs/guix-guile.el (guix-guile-keyword-regexp): New function.
* emacs/guix-devel.el (guix-devel-faces): New custom group.
  (guix-devel-modify-phases-keyword): New face.
  (guix-devel-modify-phases-keyword-regexp,
  guix-devel-font-lock-keywords): New variables.
  (guix-devel-modify-phases-font-lock-matcher,
  guix-devel-modify-phases-font-lock-pre): New functions.
  (guix-devel-mode): Adjust to add/remove font-lock-keywords.